Recently, mobile communication systems have been facing explosive increase of downlink data traffic. In order to cope with this situation, for LTE (long term evolution) based wireless communication systems, it is considered to introduce a heterogeneous network where, as shown in FIG. 1, within a coverage of a macro cell served by a Macro-eNB 10 (a first base station), a Pico-eNB 20 (a second base station) whose transmission power is smaller than that of the Macro-eNB 10 is placed and forms cell of smaller size.
In such a cell deployment, when carrier frequencies of the macro cell are the same as those of the pico cell, interference from the macro cell to the pico cell is an issue. To cope with this issue, in LTE-Advanced (3GPP LTE Release-10), in order to reduce the inter-cell interference at the same carrier frequencies, which arises between the macro cell and the pico cell placed within the macro cell, ABS (Almost Blank Subframe) has been introduced (refer to NPL 1).
ABS means a subframe for curtailing or reducing the interference from the macro cell to the pico cell. This is achieved by, for example, the Macro-eNB 10 not to transmit any piece of user data on ABS. The interference influenced on the pico cell can be reduced on a time division basis by setting some of subframes to ABS at regular intervals in the macro cell.
Meanwhile, in FIG. 1, each of the UEs (User Equipment: terminal) 30-1 and 30-2 measures a reception quality of a reference signal transmitted from each of the macro cell and the pico cell, and selects a serving cell. Such measurements of the reception qualities each regarding a corresponding one of the macro cell and the pico cell are called RRM measurements. In the LTE, each of the UEs 30-1 and 30-2 compares measured values of the reception qualities each regarding a corresponding one of the macro cell and the pico cell, and determines whether to trigger an event regarding a handover operation (for example, Event A3) on the basis of a result of the comparison. Further, upon occurrence of the event, each of the UEs 30-1 and 30-2 notifies the occurrence of the event to an eNB which is either the Macro-eNB 10 or the Pico-eNB 20 and with which each of the UEs 30-1 and 30-2 is communicating (refer to NPL 2). Upon reception of the notification for notifying the occurrence of the event from each of the UEs 30-1 and 30-2, the eNB, which is either the Macro-eNB 10 or the Pico-eNB 20, undertakes a handover operation for each of the UEs 30-1 and 30-2 on the basis of the event. In this case, the boundary between the pico cell and the macro cell is determined in accordance with the position where the event on handover operation occurs, and the position where the event occurs can be controlled by offset values each applied to a corresponding one of RRM measurements. In the deployment of heterogeneous network, the boundary between the pico cell and the macro cell tends to shift towards a pico cell side due to the interference from the macro cell, so a range of the pico cell tends to become smaller. To solve this, it is possible to adjust (broaden) the range of the pico cell to a proper one by appropriately selecting the above-described offset values.
In the case of the LTE, the reception quality measured for RRM measurements corresponds to RSRP (Reference Signal Received Power) which is a received-power level of the reference signal, or RSRQ (Reference Signal Received Quality) which is a reception quality of the reference signal.